The complex junction between Sertoli cells of the testis represents the main component of the blood testis barrier. The establishment of this barrier coincides with the initiation of spermatogenesis and effectively isolates the later stages of spermatogenesis from the interstitial fluids. How these junctions between Sertoli cells are modified to permit the ascent of the developing germinal elements from the basal tubular compartment toward the lumen is not known. In a preliminary experiment human chorionic gonadotropin (hCG) altered the Sertoli:Sertoli complex junction. In our experiments known concentrations of hCG will be perfused in vivo into the vascularly isolated testis of the anesthetized dog. The testis will be treated with fixative containing an electron-dense tracer, lanthanum nitrate, to determine the patency of the Sertoli:Sertoli tight junction. The information obtained by the tracer studies will be correlated with the results obtained by freeze-fracture studies of these junctional complexes in normal and hCG-treated adult and prepubertal dogs.